The only way I can see them casting someone white, is if the actor is non-binary. I've said before that Chibnall casting a female Doctor wasn't brave, it was the least he could do, and if the next showrunner defaults to a white actor (even if it's a woman), that will probably be a death-blow for the current era of Doctor Who because the world has changed a lot since 2017.

(and as I have also said before, they already used the "Best actor for the job" card with Capaldi, so even that defence would not work)